Global Positioning Systems (GPS) are Satellite-based radio positioning systems that provide three-dimensional position and time information to a user with the proper equipment located anywhere on the surface of the Earth. The NAVSTAR GPS system, operated by the U.S. Department of Defense, is the first GPS system widely available to civilian users.
GPS receivers often yield inaccurate positioning results due to errors that enter the system. Most of the errors are "common errors". Common errors are errors that are experienced by all the GPS receivers in a local area. Common errors include clock deviation, satellite orbit drift, selective availability, and changing radio propagation conditions in the ionosphere.
To improve the accuracy of GPS receivers, differential GPS (DGPS) was created. Differential GPS (DGPS) operates by eliminating known errors in a GPS receiver to make the results more accurate. Specifically, a GPS receiver is placed at location for which the position coordinates are accurately known and accepted. This GPS receiver at an accurately known location is called a "base station". The base station calculates the difference between the known coordinates and the GPS receiver calculated coordinates to determine the common error introduced to all GPS receivers in the local area. The common error information is referred to as error correction data. The error correction data determined by the base station can be applied to other moveable GPS receivers (known as rovers) in the local area to improve the accuracy of the rover GPS receivers.
The sources of common error are continuously changing. Thus, it is necessary to match the error correction data from the base station very closely in time to the data from the roving GPS receivers. One method of matching the data is to transmit the error correction data from the base station directly to the roving GPS receiver such that the error correction is performed immediately in the rover GPS receiver. This technique is known as real-time DGPS.
Another method of matching the error correction data with the rover position data is to record both the error correction data of the base station and the position data of the rover receiver with time-stamps. The two time-stamped data sets can then be processed together at a later time by matching the time-stamps. This technique is known as post processing.
With the post processing technique, both the base station and the rover record data simultaneously and store the data into time-stamped files. The time-stamped files must later be matched. If there is more than one base station, then the error correction files from the nearest base station must be used for the most accurate results. The process of identifying the proper time-stamped error correction files from the nearest base station is time-consuming task that is error prone. It would therefore be desirable to have an improved method of identifying the error correction files needed for differential GPS post processing.